Very large scale integrated circuits, also called VSLI chips, currently dissipate a major amount of heat over a small surface area. For example, N-MOS integrated circuits are capable at present of dissipating on the order of 5 watts across a surface of approximately 1 square centimeter. An N-MOS integrated circuit is a small semiconductor plate, the active components of which are N-channel metal oxide semiconductor field effect transistors, or N-MOSFETs. Each integrated circuit is assembled in a package provided with a heat exchanger in order to remove calories. The invention relates to heat exchangers including a metal-to-air interface, which are currently known as heat sinks. At the present time, a heat sink for a VSLI package is typically in the form of a solid column of material that is a good thermal conductor, provided with fins in the form of concentric rings regularly spaced along the length of the column. In complex electronic machines, such as information processing machines, the packages are connected electrically by disposing them on one or more printed circuit boards. Considering the small size sought for each VSLI package, a board may include a relatively high number of packages quite close together. The problem then becomes to remove a very large quantity of heat, concentrated on a board of small surface area, by ventilating the heat sinks.
In the prior art, cooling of the low power or less-dissipating components arranged in rows on a substrate or board is done by serial ventilation of the components of each row. The cooling device is simple, but it has the disadvantage that the last components of the row are ventilated with the air that has been heated by contact with the preceding components. Various improvements have been made, in particular an improvement in which the flow of cooling air is accelerated in proportion to how highly heated it becomes, by causing the air to pass through a conduit having a smaller and smaller cross section. With highly dissipating or high-power components, the flow of air heats very quickly from contact with the first components and is no longer suitable for cooling the following components. Thus the output of air must be increased considerably, and its speed must be increased by means of very powerful and very noisy fans. It will be appreciated, however, that the noise level must be as low as possible, especially for office information processing machines.
One solution for cooling high-caloric-dissipation components has been disclosed in French Patent No. 2 157 094, corresponding to U.S. Pat. No. 3,843,910. The principle of this solution is to ventilate each component separately with the same fresh incoming air. In one embodiment, the fresh air inlet conduit envelops the face of the substrate that carries the components, and the hot air outlet conduit covers the other face. The inflowing fresh air is thus put into direct contact with all the components and exits via holes made in the substrate so that it can be evacuated into the outlet conduit.
This embodiment has two major disadvantages. First, it is complicated to assemble the two conduits on the two respective faces of a printed circuit board. The unit is quite thick, and it becomes difficult to install it in a machine. On the other hand, it is clear that to remove powerful thermal forces requires high-powered ventilation. The holes made in the board for the passage of hot air into the outlet conduit must accordingly have a large cross section. The presence of equally large holes is to the detriment of the concentration of the printed circuits in the board and causes the loss of the advantage of being able to use boards having a small surface area. The cross section of the holes can be reduced, but smaller holes increase the resistance to the flow of air. This resistance must be compensated for by higher power on the part of the ventilation source, which thus becomes noisier. Contrarily, this embodiment is independent of the disposition of the components among one another and does not require disposition of the components in rows.